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Unformatted text preview: -nnouncements W cumulative '.j:_'with whatever they want handwritten on it. (Yes, you 7 -oth sides.) ,I #7 assigned NASA1 NOAO, ESA and The Hubble Heritage Team (STScl/ Interstellar Medium (Kutner, Ch. 14; also Shu, Ch. 11, Carroll and Ostlie Ch. 12) ASTZDS (Spring 2011) LastThne:Exfincflon Compare a star's apparent magnitude with what we expect from its spectral type (which tells us L) and distance: observed extinction expected We saw that: A N T The magnitude relation now becomes: m = Jl/[+ 5log(d/10 pc) + A If we consider 2 different wavelengths (filters): (m3 — mv) = (MB — Mv) + (AB — AV) w a This is observed This is B-V and is determined by the spectral type AST 203 (Spring 2011) Exflncfion We can use this behavior to do spectroscopic parallax to tell us the distance to a star—we just need to correct for the dust. Consider a BS star: My : —0.9, B — V : —0.17 We observe mB : 11, my : 10 Starting AB — AV : (mB — my) — (MB — MV) AB — AV : (11 — 10) — (—0.17) : 1.17 What do we do now? ASTZDS (Spring 2011) Exflncflon A useful quantity is the ratio of total-to-selective absorption: AV R—ABZAV Since both AV and A3 are oc N, the column density drops out. This is independent of the amount of dust along the line of sight. We might expect R to be roughly the same all over the sky It doesn't matter how much dust we are looking through. Observations tell us thatR N 3,1 R is determined by the nature of the dust. AST 203 (Spring 2011) Exflncfion Using Av -R:thz; we have AV Z — AV) Z ' Z Then my : My —l— 5 log(d/10 pc) —l— AV d : 10 pc - 10(mV—Mv—AVV5 : 10 pc . 10(10—(—0.9)—3453)/5 : 285 pc What would happen if we did not account for the extinction? ASTZDS (Spring 2011) Without Extinction Using mv = Mv + 5log(d/10 p0) d : pc _ 10(TTLV—Jl/Iv)/5 : pc . 10(10_(_0'9))/5 = 1510 pc ASTZDS (Spring 2011) Polarization Light passing through clouds is often polarized. Star light is not polarized—the cloud is doing the polarization. Polarization needs alignment of the dust grains. Scattered/absorbed light will be preferentially polarized. Low AV = low polarization. What causes the dust to align? Probably magnetic fields. AST 203 (Spring 2011) Albedo Extinction is from scattering and absorption—which is more important? This depends on the dust and the wavelength. The fraction of light that is scattered is called the albedo, a)“ The fraction that is absorbed is then 1 — aA. Albedo can be estimated from reflection nebula. This tells us that a), N 0.5 — 0.7 - A photon is more likely to be scattered than absorbed. AST 203 (Spring 2011) Dust Grain Size The composition, size, temperature, and charge of the dust grains determines what we see. Consider grain size r If 7" >> A : grains simply block the photons that strike them. A()\) is constant If r << A : photons don't see the grains —> very little extinction. We can see right through the clouds at these wavelengths. If 7" ~ A : diffraction becomes important. A()\) varies a lot in this range AST 203 (Spring 2011) Extinction | ' | ' | ' | ' SMC Bor — 6 — LMCZ Supershell — a; - —+— LMC Averoge — 3 - — ---- -- MW (RV = 3.1) - f; :t’ 4 — griii" 7: \ ‘ D Q ' . ijlryfi ' x' V . -" i ,-" - S < _ ‘fll‘tt:;_055" _ E 2 — _ éa - fix - - x' - _ _,-/ _ a; . E0 of. I I I I I I I I I I I I I I _l E9 2 4 6 8 1/>\ [Mm—1] Extinction as a function of wavelength. More complicated than our simple arguments—bump suggests graphite composition (resonance at 217.5 nm) ASTZDS (Spring 2011) ISM Summary The Dark Cloud B68 at Different Wavelengths (N TI' + SOFI) ~ E50 PR Photo 29M)? I ljuly 1999) © ELli'l Ipezln Sntlthem Obxcn'mnw 4' ASTZDS (Spring 2011) Visible ' ' A _ .' - ' _' infrared Visible vs. Infrared View of Pillar and Jets HH 901/902 Hubble Space Telescope o WFCS/UVlelR NASA ESA and M Limo and the Hubble Efllh Anniver‘saaneam (STScli STSCl-FRCIO-nb Dust Grain Size Different types of dust grains have different average sizes. Polarization shows that the grains are elongated or disk shaped. Absorption features from the grains tell us the composition SiO, SiO2 and water ice have vibrational lines at 10 and 12 pm UV extinction shows the presence of C The small grains are ~ 1 nm in size and consist of 20-100 C atoms—polycyclic aromatic hydrocarbons. Typical PAH (lnductiveloadlleipedia) ASTZDS (Spring 2011) Dust Grain Size The larger grains have a more complicated, layered structure. Together, these two types of grains explain the observations. Dust is only a small component of the ISM—gas accounts for most of the mass. ASTZDS (Spring 2011) Temperature of Dust Grains We can compute the dust grain's T, at a distance away from a Star. 13* Z X T . Ty __ J. d The luminosity of the star is L* : 47eran 13* 47rd2 The flux at the grain is then f I If the albedo is a, then what is the power absorbed? L* 2 R3 anm": 7T7“ — 47rd2 g d2 PabS=(1—a) =(l—a) AST 203 (Spring 2011) Temperature of Dust Grains Ifthe grain is in thermodynamic equilibrium with its surroundings, Pabs : rad The grain emits as a blackbody, so and then Note, if the albedo is 0, then we are a perfect blackbody. Note: your book includes an albedo factor in the radiated power. The wavelength that the grain absorbs at is different from the wavelength that it emits at—these albedos can be very different. ASTZDS (Spring 2011) Temperature of Dust Grains Consider a grain 5000 stellar radii away from a star whose surface T is 10000 K. Assuming the grain albedo to be 0.5, R Tg : (1 — 0.5)”4 10 000 K m : 80 K This is a good typical temperature for a dust grain. Note, that this is hot enough to emit in the infrared. ASTZDS (Spring 2011) Evolution Dust Grains Dust grains may form in red giant outer envelopes. These stars undergo mass loss As material leaves and cools to ~1000 K, silicates can form. This material is blown into the ISM by the stellar wind or during the planetary nebula phase Grains in clouds can accrete from gas and grow in size. Grains can also get smaller through sublimation or via collisions. ASTZDS (Spring 2011) Interstellar Gas Dust is ~ 1% of the ISM mass. ISM is mostly gas (primarily H) This can be neutral (H I), ionized (H II), or molecular (H2) Most of the H is neutral. At low ISM T, electrons are mostly in the ground state. No emission lines Absorption lines require the presence of UV photons Absorption lines have narrow widths (differ from stellar lines). Remember, the width of the line is due to Doppler broadening. ISM gas is much cooler than stellar atmospheres. ASTZDS (Spring 2011) Interstellar Gas We observe neutral H via the flip of the spin of the electron. This produces a line at 21 cm We can observe this easily in the radio. This line can be excited with ? temperatures as small as ~0.05 K (based on an image fi'om Wikipedia) Line predicted to exist in the ISM in the 19403 by van de Hulst. Purcell (1950) detected it and won a Nobel prize. AST 203 (Spring 2011) Exciting the 21 cm line What temperature is needed to excite the 21 cm line? Recall that temperature is a measure of the average kinetic energy of particles. 3 Thermal energy: E = ng 3 he —l<;T= — 2 )x T _ 2 he _ 2 6.67 X 10—27 erg s - 3 x 1010 cm 8—1 _ __ _ = 0.05 K 3 Ak 3 2lcm- 1.38 X 10—16 erg K—l ASTZDS (Spring 2011) Interstellar Gas J. Dickey (UMn), F. Luckman (NRAO), http://EWifiYésfcnasa.gov/apod/apm 01 1 3.html An all-sky map of neutral hydrogen. ASTZDS (Spring 2011) Interstellar Gas 21 cm observations tell us the location and amount of interstellar H I, the radial velocities of the H I, and magnetic field strengths (via the Zeeman effect). Interstellar dust is transparent in 21 cm (since A > rg) We can see clouds all over the galaxy. This tells us about the structure of the galaxy. Typical H | clouds have T ~ 100 K For comparison, in our nH ~ 1 _ 10 cm-3 atmosphere, n ~ 1019 cm'3. R ~ 10 pc The best laboratory vacuums reach n ~ 1010 cm'3. ~ 21 —2 NH 10 cm AST 203 ISpring 2011 I Interstellar Gas These clouds account for ~ 5% of the ISM. Between the clouds we see evidence for hot, rarefied gas, with temperatures ~ 1000 - 10000 K. highl Hum .ommuum I mun-c intercloud imcmloud \ mudium medium (Shu) fi— \ .000 4r— ‘ mini-mum ASTZDS (Spring 2011) Interstellar Gas Between the clouds, density is low —> difficult for the gas to cool. Two-phase model for ISM: dense, cool H | clouds in pressure equilibrium with the hot rarefied intercloud medium. Pcloud : PIC ncloud Tcloud : nIC TIC Inter-cloud densities are Tcloud _3 102 K : 10 TIC cm 104 K Nature is far more complex however. 3 n10 : ndoud : 0.1 CH1— There are also million K bubbles of gas heated by supernovae. Our Sun is currently inside one of these bubbles. ASTZDS (Spring 2011) ASTZDS (Spring 2011) Table 19.1 Typicll Status 6! Gas in tin intent-1hr Medium ISM Summary Hnl bubbles Warm atomic gas Cool atomic clouds Molecular clouds Molecular cloud cores (Bennett et al.) [unified LilliflJil'lll l< hydrogen Atomic [0.000 K hydrogen Alumld I00 K hydrogen Molecular 30 K hydrogen Molecular Gil) K hydrogen mum emu mm (mm wmhng as mu wwgy, 'Pocktls nfgas healed hy supmmva shock wavus l‘ills much afgalacric disk Intermediate stage of smr—gaksmr cydn Regionsof star lurmdliun Starel’orming clouds Interstellar Gas 21 cm emission is coincident with optical extinction The gas and dust are mixed. From 21 cm emission, we see that NH is correlated with visual extinction, AV , for AV < 1 ForAV > 1, H combines to form H2. Since NH is high, there is shielding from UV photons that would have dissociate H2. H2 does not emit in 21 cm, so it is not seen. AST 203 (Spring 2011) (Kutner) Visual mmcuan A, Thaugh mere 15 some smut-r, mm .s a good correianm in um [wn qua than. [mu magnitude minus ,1; lung “A, is less Local Bubble Linda Huff (American Scienlt), Priscilla Flisch ( Mtfimflfifpgsfonasa.gov/apod/apDZDZ1D.html Imam .maegas Linda Huff (Amelican Scientist), Pliscilla Frisch ( Mfimrp.gsfc.nasa.govlapodlap02021 7.html A model of the local ISM. The Sun is believed to moving through the local interstellar cloud just on the edge of the local bubble. ASTZDS (Spring 2011) Interstellar Molecules Interstellar clouds classification: H | clouds: mostly neutral hydrogen H II regions: ionized hydrogen (we'll see these in more detail when we look at star formation) Molecular clouds: H in molecularform (H2) Lots of different molecules were discovered early-to-mid 20th century (including CH, CH”, and CN). Here, molecules = short lengths of atoms (~ 7-10 atoms) How likely is it to form molecules? ASTZDS (Spring 2011) Interstellar Molecules H l clouds have low density —> formation rate of molecules is low. To form molecules, atoms need to get together. Consider CO: no = number density of C, no = number density of O U = cross-section for interaction For 0 atoms moving with speed 2), formation rate is: Rform : (TI/CU) (n00) : TLCTLQO'U If we take 710 N no N 10—3 “H (solar abundances) 0' N 10—16 cm2 (typical atomic geometric cross-section) v N 105 Cm 8—1 (thermal speed for T ~ 100 K) and take 71H N 10 CHI—3 (H l cloud), we have Rform : 10—15 CHI-3 5—1 AsT 203 (Spring 2011) Interstellar Molecules Equilibrium #: balance formation and destruction rates UV photons can break apart the CO molecules Typical lifetime in a H l cloud is ~ 103 years SO Rdest : nCO/tdest and Rform : Rdest therefore nCO : Rformtdest nco : 10—15 cm—3 8—1 - 103 yr - 3.16 X 107 s y1r—1 : 3 X 10—5 cm—3 This is an incredibly small number density. OH, H20, and NH3 discovered in radio In 1969, CO was detected at A = 2.6 mm. The amount of CO was inferred to be ~ 1 cm—3 What accounts for this discrepancy? AsT 203 (Spring 2011) Interstellar Molecules Answer: molecules don't form in the H l clouds we see in 21 cm. Instead they form in denser objects called molecular clouds. Higher density means higher formation rate Also means more shielding from UV photons We think that some molecules form on interstellar grain surfaces This increases the rate of molecule formation as well We observe rotational modes—changes in the rotational angular momentum of the molecule. ASTZDS (Spring 2011) J. Dickey (UMn), F. Luckman (NRAO), Sk V' http:/Iant‘vr'gfvgsfc.nasa.gov/apod/apm D1 1 3.html All-sky map of neutral hydrogen. T. Dame (CfA, Harvard) el al., Columbia 1.2-m Radio M o I e cu Ia r ma p Talesmpfifipjlantwrp.gsfc.nasa.gov/apod/ap970430html ASTZDS (Spring 2011) ClEdil: Hubble Heritage Team (STScl/AURA), N. Walbum (STScl) a R. Bale (La Plala Obs), NASA http://antwrp.gsfc.nasa.gov/apod/ap030630.html Molecular cloud near Carina nebula. About 2 ly across, It is being eroded by radiation from nearby young stars. ASTZDS (Spring 2011) ASTZ Interstellar Molecules W— 2 3 4 5 r 8 9 H) II I} AIF C; C-CjH C5 Cw Chi-i 041cm Cl quil'l AIC CH IVCV.H CH CHO CHCHCN HCOGCH, Cll=Cill;CN C; QC! CJN C45. cg—i.‘ CHQH CgH‘ CH C5 {40 [Hr CHJCN HCgN ctHT CH‘ CH. cjs :rC:HA, Cl—leC HCOCH. CH CN l—lCN CJH: CH_.I:N CHJOl-l NHfil—l, co HCO CHJD‘ 0—1:; CHJSH cw; CO' HCO l—lCCN HQN HCNH‘ Ethyl alcohol CF HCS' HCMH’ HCINC HCOCH, cs HOC' HNCC HCOOH HCONH; cal H;O l-NCS H__CHN Hp l-lCl H73 HOCO' HCO cap H: em HJCO H_NCN mi mo HACN HNQ M l MgCN was siHi no Mch H m CH 30' NS we Nil. MCI NH) l lOCS OH NO cw, PN NaCN C5 so OCS 59' 50, SN use 50 CO;- sis C0; Cl—l' HN5CQ.‘ HF up Ill»: H; ()H' HS N} (Kutner) Observing Interstellar Molecules Emission can occur via: electrons —3 jumping from one level to another (like in an atom), rotation, or vibration. Energy level jumps require ~ eVs— hard to excite. Vibration transitions (think of springs) are typically in IR. Rotation energies are low—can be excited in ISM. 1 l I) Energy —Iv H2: no lines in visible or radio Hd—l CO acts as a tracer m u. 3 1 I ll Fig ILIS. Relation] mar” lands [or twin diammic Rotational angular momentum is Waco.“a.n,...t...n.m..n.a.m ' rmzlz'unal quantum ntlnburjjha difllrunnu between the I mmolemlesarlmlmmdaedlflermcuhmuflanllmrfiu milking [rum diflurunt masses [or 0 and 5. and diflnmm: I hand W! bump»: mole:u_|_e§ AST 203 (Spring 2011 ) (Kutner) Observing Interstellar Molecules CO abundance is ~ 10'4 to 10'5 H2 abundance Most other molecules have abundances 10'9 of H2 Mass determinations of H2 clouds rely on indirect tracers, not the primary component True ratio of CO to H2 may vary due to thermodynamic conditions. Not a problem with HI clouds—there we measure primary component (through 21 cm), can convert column density into mass. Can still estimate virial mass (if we assume cloud is relaxed). ASTZDS (Spring 2011) ISM Thermodynamics ISM temperature from balance of heating and cooling Heating processes: - Heating dust: dust absorbs photon, grain heats. Grain transfers energy to atoms/molecules via collisions - Excitation of atoms/molecules: photon strikes molecule/atom directly, exciting it. Energy converted to KE of second atom/molecule through collision. - Ionization: photon ionizes atom/molecule. Free electron gives KE to gas via collisions. - Photoelectric effect: photon strikes dust grain, ejects electron. Electron KE heats gas. ASTZDS (Spring 2011) ISM Thermodynamics Cooling processes: - Grain emission: grain heats from collisions with atoms/molecules. Radiates - Excitation: atom/molecule collisionally excited, emits a photon before collisional de-excitation. - Ionization: collision between atoms/molecules ionizes one. Electron recombines emitting a photon (recombination before electron can collide with atom/molecule) ASTZDS (Spring 2011) ...
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